This invention relates to a projection display device that magnifies and projects the image displayed on a scattering-type liquid crystal light valve onto a screen, and to a light source device that is incorporated into the projection display device etc.
The main type of the liquid crystal light valve used in projection light valves has consisted of twisted nematic (TN) liquid crystals sandwiched between two polarizing plates. In such a projection display device, however, since over half of the light emitted by the light source device is lost at the incident-side polarizing plate, and since, to avert thermal degradation of the polarizing plate due to light absorption, it is necessary to limit the light emitted, it was impossible to increase the brightness of the projected image.
A projection display device that dispenses with the polarizing plate and uses a scattering-type liquid crystal light valve is disclosed, for example, in Japanese Patent Kokai Publication No. 188345/1993 (H5-188345). The scattering-type liquid crystal light valve may be, for example, a polymer-dispersed liquid crystal (PDLC), or a dynamic scattering mode (DSM) liquid crystal. FIG. 1A and FIG. 1B are explanatory diagrams showing the principle of the PDLC. In the figures, the PDLC comprises a polymer 205 sealed between transparent substrates 203 and 204, which are respectively provided on the inner surface with electrodes 201 and 202, and liquid crystal 206, which is dispersed therein in the form of droplets. When voltage V is not applied between the electrodes 201 and 202, the liquid crystal molecules 206a within the liquid crystal 206 are oriented in random directions, as shown in FIG. 1A, with the result that a difference in refractive index arises between the polymer 205 and the liquid crystal 206, and incident light 207 becomes scattered light 208. On the other hand, when voltage V is applied between the electrodes 201 and 202, the liquid crystal molecules 206a of the liquid crystal 206 are oriented in the direction of the electric field. Since the refractive index of the liquid crystal 206 and the refractive index of the polymer 205 have been so chosen as to be identical when the liquid crystal molecules 206a are oriented in this way, the incident light 207 passes though in a straight line without being scattered.
However, while a projection display device provided with a scattering-type liquid crystal light valve has the advantage of being able to achieve high brightness of the projected image, it suffers from other problems to be described blow.
The first problem occurs in projection display devices of the type shown in FIG. 2. In such a projection display device, emitted light 214 from a light source device 213 comprising a lamp 211 and a parabolic mirror 212 passes through a scattering-type liquid crystal light valve 215, a field lens 216 and a projection lens 217, so as to magnify and project it onto a screen 220. Then, to increase the contrast of the projected image, the scattered light component of the liquid crystal light valve 215 is removed by varying the aperture diameter of a diaphragm 218 of the projection lens 217 to reduce the diameter of an entrance pupil 219. In the event, however, that the lamp 211 has luminous region with long and narrow shape, such as a metal halide lamp or halogen lamp, rather than luminous region in the shape of dot, such as a xenon lamp, even if the aperture diameter of the diaphragm 218 is varied and the diameter of the entrance pupil 219 is reduced, the brightness of the projected image merely decreases without any adequate increase in contrast.
The second problem occurs in the projection display devices of the type shown in FIG. 3, using dichroic mirrors 35a and 35b and a dichroic prism 38. In such the projection display device, emitted light 100, which was emitted by a light source device 33 comprising a lamp 31 and a parabolic mirror 32 and passed through a filter 34, is separated by the dichroic mirror 35a into red light 100R, which passes through, and green light 100G and blue light 100B, which are reflected, and the reflected green and blue light is separated by the dichroic mirror 35b into the green light 100G, which is reflected, and the blue light 100B, which passes through. The red, green and blue homogeneous light 100R, 100G and 100B then pass through liquid crystal light valves 37R, 37G and 37B respectively, and impinge from each incident surface for homogeneous light onto the dichroic prism 38. Composite light consisting of the red, green and blue homogeneous light 100R, 100G and 100B impinges on a projection lens 39, and is magnified and projected on the screen 12.
In terms of the transmittance T, the dichroic mirror 35a has a wider transmission band of p-polarized light component than s-polarized light component, as shown in FIG. 4A. In terms of the reflectivity R, the dichroic prism 38 has a wider reflectivity band of s-polarized light component than p-polarized light component, as shown in FIG. 4B. Thus with respect to red light, s-polarized light component is limited by the dichroic mirror 35a, while p-polarized light component is limited by the dichroic prism 38. As shown in FIG. 4C, the integrated spectral characteristic T.times.R becomes narrower for both p-polarized light component and s-polarized light component, so that the efficiency of light utilization is low.
The third problem occurs in the projection display devices using dichroic mirrors 85a, 85b, 85c and 85d, as shown in FIG. 5. In such the projection display device, light 100, which was emitted by a light source device 63 comprising a lamp 61 and a parabolic mirror 62 and passed through a filter 64, is separated by the dichroic mirror 85a into red light 100R and green light 100G, which pass through, and blue light 100B, which is reflected. The red and green light are separated by the dichroic mirror 85b into red light 100R, which is reflected, and green light 100G, which passes through. The blue light 100B passes through a liquid crystal light valve 87B and a field lens 88B, then passes through the color-synthesizing dichroic mirrors 85c and 85d, and impinges on the projection lens 69. The red light 100R passes through a liquid crystal light valve 87R and a field lens 88R, is reflected by the color-synthesizing dichroic mirror 85c, passes through the color-synthesizing dichroic mirror 85d, and impinges on the projection lens 69. The green light 100G passes through a liquid crystal light valve 87G and a field lens 88G, is reflected by a reflecting mirror 86b and the color-synthesizing dichroic mirror 85d, and impinges on the projection lens 69. Thus homogeneous light impinged on the projection lens 69 becomes a single composite light, and the composite light is magnified and projected on the screen 12.
For example, with regard to the blue light, the dichroic mirror 85a has the reflectivity R shown in FIG. 6A, and the reflected blue light is partially polarized in such a way that s-polarized light component is a wider band than p-polarized light component. The dichroic mirror 85d, which has the function of allowing the blue light to pass through, has the transmittance T shown in FIG. 6B, and the transmitted blue light is partially polarized in such a way that p-polarized light component is a wider band than s-polarized light component. Thus the integrated spectral characteristic R.times.T of the dichroic mirrors 85a and 85d, are as is shown in FIG. 6C. Thus, with respect to the blue light, the s-polarized light component is limited by the dichroic mirror 85d, while p-polarized light component is limited by the dichroic mirror 85a, so the efficiency of light utilization is low.
Further, there is a trend away from the use of halogen lamps as the light source device in the projection display devices and toward the use high-voltage discharge lamps such as short-arc type compact metal halide lamps, which offer greater light output, higher luminous efficiency, and longer lamp life, along with outstanding color rendition. FIG. 7 is a perspective view showing the structure of a light source device having a high-voltage discharge lamp which was proposed in Japanese Utility Model Kokai Publication No. 29902/1991 (H3-29902). This light source device comprises a concave surface of revolution, which may be a spherical, parabolic, elliptical or other surface, and which has a notched reflecting mirror 101, a metal halide lamp 102, a mouth piece 103 to hold the lamp 102, and a mounting piece 104 to hold the mouth piece 103 on the reflecting mirror 101. Thus, the flow of air in the direction of the arrows in the figure is improved by the notch, thereby increasing cooling efficiency. In addition, FIG. 8 is a cross-sectional view showing a light source device provided with an air duct 117, which is disclosed in Japanese Patent Kokai Publication No. 127138/1992 (H4-127138). In this light source device, an air stream is delivered from the duct 117 in the direction shown by the arrows using a fan (not shown), and cooling air is delivered at the inner side of the reflecting mirror 101.
The light source devices of FIG. 7 and FIG. 8, however, suffer from the problem that air cannot be delivered equally at the periphery of the lamp 102 so that, as a result of irregularity in the surface temperature of the lamp 102, emission distribution becomes uneven. Further, the light source device in FIG. 8 has the problem that the device is larger.